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373 
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IRLF 


THE  OPTICS  OF  METALLOGRAPHY 


By  W.  L.  PATTERSON 


BAUSCH  &  LOME  OPTICAL  CO. 


of  California 

593  Market  Street 

San  Francisco,  California 


1  IDftA 


NOTE  : — Practically  all  of  the  important  developments 
in  equipment  for  metallography,  since  the  first  publica- 
tion of  this  article,  are  embodied  in  the  new  Bausch  & 
Lomb  Large  Metallographic  Equipment.  For  this  rea- 
son, a  rather  complete  description  of  this  apparatus  has 
been  incorporated  in  this  revision. 

In  order  that  no  confusion  may  exist  as  to  which 
equipment  is  referred  to,  all  descriptions  and  illustra- 
tions of  this  new  apparatus  are  marked  with  an  asterisk, 
thus(*). 

This  is  a  reprint  of  an  article  presented  to  the  American  Society 
for  Steel  Treating  (Vol.  II,  No.  2,  1921)  with  additions  and  revisions. 


THE   OPTICS   OF   METALLOGRAPHY 

By  W.  L.  PATTERSON 

OBJECTIVES 

The  objectives  undoubtedly  constitute  the  most  important  part  of  the 
microscope.  Their  quality  determines  the  final  results. 

The  objective  is  so  called  because  it  is  nearest  to  the  object.  Figures  i, 
2,  and  3  show  respectively  the  16,  4,  and  1.9  millimeter  objectives  of  the 
usual  type,  but  they  are  made  in  a  variety  of  focal  lengths,  numerical  aper- 
tures, and  styles  of  .mount  by  various  makers.  Following  are  given  the 
usual  focal  lengths  and  numerical  apertures  (N.  A.)  of  achromatic  objec- 
tives: 

Focal  Lengths         Numerical  Type  of  Lens 

Millimeters    Aperture  (N.A.) 

32  o.io  Sirigle  achromatifc  lens. 

1 6  °-25  Two  achromatic. doublets. 

8  0.50 


4  0.85 

3  0.85 

1.9  1.25 

1.9  1.32 


These    three    have   non-achromatic    front    with    2 
achromatic  doublets.   6  lenses  in  all. 

{ Fluorite  construction,  semi-apochromatic. 

Further  reference  will  be  made  to  the  apochromatic  objectives  later. 
The  focal  length  of  an  objective  does  not  indicate  its  working  distance,  but 
means  that  the  combination  of  lenses  composing  the  objective  is  equal  in 
focus  to  a  single  lens  of  the  stated  focus.  Thus  the  working  distance  for 
the— 

1 6     millimeter  focus  =  7       millimeters 
4  "        =  0.30 

1.9  "        =  0.15 

An  objective  may  be  said  to  possess  seven  qualities:  magnifying  power; 
aperture,  or  numerical  aperture;  resolving  power;  depth  of  focus,  or 
penetrating  power;  illuminating  power;  flatness  of  field;  and  defining 
power.  To  fulfill  all  of  these  requirements  in  a  satisfactory  manner 
necessitates  considerable  skill  upon  the  part  of  the  optician.  These  quali- 
ties will  be  discussed  one  by  one. 

Magnifying  power  usually  is  stated  in  the  catalogues  of  various  makers 
in  terms  of  the  combination  of  a  certain  objective,  certain  eyepiece,  and 
tube  length  at  a  predetermined  image  distance.  While  tables  given  in  the 
catalogues  are  approximately  correct,  the  magnification  stated  will  be  ob- 
tained only  if  the  conditions  given  are  fulfilled.  If  the  tube  length,  that  is 
the  distance  between  objective  and  eyepiece,  is  increased  or  decreased,  or 
an  increase  or  decrease  in  image  distance  is  taken  as  in  photography,  the 
magnification  tables  will  not  prove  correct.  Tables  given  in  catalogues 
are  often  for  visual  work  and  are  based  on  an  apparent  or  virtual  image 
being  formed  in  space  at  a  distance  of  250  millimeters  from  the  eye.  This 

Urn  i  ion  A 


Fig.  i — 1 6  mm  objective  of  usual  type 

Fig.  2 — ^  mm  objective  of  usual  type. 

Fig.  3 — /.<?  mm  objective  of  usual  type. 

apparent  size  will  vary  with  persons  having  near — or  far-sighted  eyes,  but 
the  normal  distance  is  taken  as  250  millimeters  and  the  size  of  the  image  is 
the  same  if  projected  on  the  ground  glass  of  a  camera  at  250  millimeters. 

Makers  of  objectives  also  give  in  their  catalogues  the  initial  magnifica- 
tion of  their  several  objectives,  that  is  the  magnification  they  give  at  a 
certain  distance  unaided  by  the  eyepiece.  These  tables  of  initial  magnifi- 
cation will  be  found  to  vary  in  different  catalogues  even  though  objectives 
are  of  the  same  focal  length,  this  being  due  to  the  different  systems  of 
calculating  magnifications.  It  will  be  found,  however,  that  for  the  same 
focus  of  objective,  the  same  focus  of  eyepiece,  and  the  same  tube  length, 
the  figures  nearly  agree.  Some  makers  use  a  number  system  for  both  eye 
pieces  and  objectives,  but  in  making  comparisons  only  focal  lengths 
should  be  considered. 

In  the  large  inverted  forms  of  metallurgical  microscopes  the  distance 
between  objective  and  eyepiece  is  usually  about  40  millimeters  longer 
than  in  the  ordinary  table  microscope,  and  therefore,  the  magnification  is 
increased  some  25  percent  even  with  the  same  focus  of  objective  and  eye- 
piece; hence  the  difference  in  magnification  tables  for  this  microscope. 
With  the  many  variable  factors  in  different  outfits  it  is  difficult  to  make 
comparisons,  and  it  is  best  to  calculate  the  magnification  for  the  particu- 
lar outfit  being  used,  especially  as  regards  camera  magnifications.  To  ob- 
tain the  exact  magnification  at  the  ground  glass  of  the  camera,  and  there- 
fore of  the  photograph,  it  is  best  to  project  a  stage  micrometer  upon  the 
ground  glass  and  measure  its  magnified  image.  Micrometers  graduated 
upon  metal  may  be  obtained  for  this  purpose.  The  rulings  are  in  tenths 
and  hundredths  of  a  millimeter.  They  are  placed  upon  the  stage  and  il- 
luminated the  same  as  a  metal  specimen.  After  focusing  upon  the  ground 
glass  the  magnification  can  be  measured  directly  without  further  calcula- 
tion. 

The  aperture,  or  numerical  aperture  (N.A.)  is  a  property  to  which 
many  buyers  of  microscopes  probably  pay  little  attention,  but  it  is  never- 
theless an  important  quality  of  an  objective.  The  diagram  in  Figure  4 
is  shown  in  an  attempt  to  simplify  the  theory  of  numerical  aperture.  If  we 


assume  that  few,  if  any,  objects  are  perfectly 
smooth,  then  light  is  dispersed  from  every 
point  on  the  surface  of  an  object  in  all  direc- 
tions up  to  1 80  degrees.  Only  an  extremely 
narrow  pencil  of  this  can  be  received  by  the 
unaided  eye.  The  apparent  problem  of  prac- 
tical optics  is  to  be  able  by  means  of  lenses  to 
gather  and  bring  to  a  focus  as  many  of  the  un- 
admitted rays  as  possible.  In  early  investiga- 
tion it  was  found  that  the  objective  which 
gathered  in  the  greatest  angle  of  rays  gave  the  AM- ""  n  3in  u' 

greatest  resolving  power.  Fig.    4— Diagram    showing 

Tr      i     j         i  •       •  i  -11          theory   of  Numerical  Aper- 

If  only  dry  objectives  were  to  be  considered,  ture 

we  might  make  a  comparison  on  the  basis  of 

angular  aperture,  that  is  the  angle  of  the  cone  of  light  which  is  admitted 
to  the  front  lens  of  the  objective  and  reaches  the  eye.  This  is  important, 
as  internal  diaphragms  reduce  apertures,  but  these  apertures  may  be 
measured  readily  by  special  instruments. 

A  thorough  study  by  Professor  Abbe,  however,  proved  that  the  only 
exact  method  for  comparison  of  objective  apertures  was  by  comparison  of 
the  sines  of  the  extreme  admitted  radiant  pencils,  and,  when  the  media 
differed  between  the  object  and  the  objective,  of  the  refractive  indices  of 
those  media,  which  are  i.oo  for  air,  1.33  for  water,  and  1.50  for  cedar  oil. 
It  is  very  evident  that  rays  emanating  from  an  object  and  passing  to  the 
objective  will  travel  in  different  angles  according  to  the  media  through 
which  they  pass,  owing  to  the  different  refractive  indices  or  bending 
powers  of  such  media. 

In  Figure  4  is  shown  a  ray  Rx  leaving  the  object  at  an  angle  of  30  de- 
grees, passing  through  air  and  just  entering  the  extreme  edge  of  the  front 
lens  of  the  objective.  A  second  ray  R2  leaving  the  object  at  the  same 
angle  and  passing  through  oil  will  be  bent  so  as  to  fall  within  the  edge  of 
the  objective  lens,  as  shown  by  R3.  It  is  obvious  that  a  ray  leaving  the 
object  at  a  still  greater  angle  than  R2  and  passing  through  oil  will  be  bent 
so  as  to  pass  into  the  objective.  Thus  a  wider  angle  of  rays  is  collected  in 
case  of  the  oil-immersion  objective  than  in  the  dry  objective. 

It  will  be  seen  that  angular  aperture  can  not  be  a  comparative  measure 
for  the  two  kinds  of  media.  Abbe  found  that  a  value  could  be  expressed 
for  the  different  conditions  by  taking  the  sine  of  one-half  the  angle  of 
aperture  and  multiplying  it  by  the  refractive  index  of  the  media  between 
object  and  objective;  hence  the  formula: 

Numerical  aperture  (N.  A.)   =  n  sin  u 

where  n    =   refractive  index  of  media:  air  i.oo,  water  1.33,  oil   1.50, 
and  u  =  one-half  the  angular  aperture. 
Thus  the  numerical  aperture  on  the  air  side  would  be 

i.oo  X  sine  30°  =  i.oo  X  .5  =  0.50 
and  on  the  oil  side 

1.50  x  sine  30°  =  1.50  x  .5  =  0.75 

3 
3  31  /O 


This  gives  a  method  of  comparison  between  dry  -  and  oil  -  immersion 
objectives. 

The  resolving  power,  which  is  the  property  by  which  an  objective 
shows  distinctly  separated  two  small  elements  in  the  structure  of  an  ob- 
ject, is  directly  proportional  to  the  numerical  aperture;  thus  an  objective 
of  0.50  numerical  aperture,  as  in  the  8  millimeter,  has  twice  the  resolving 
power  of  the  16  millimeter,  or  0.25  numerical  aperture. 

If  a  very  narrow  central  pencil  is  used  for  illumination,  the  finest  detail 
that  can  be  shown  by  the  microscope,  with  high  enough  magnification,  is 
equal  to  ^  where  lambda  is  the  wave  length  of  the  light  used  for  il- 

N.A. 

lumination,  say  one-half  micron.  The  wider  the  pencil  used  for  illumina- 
tion, the  greater  the  resolving  power  until  a  maximum  is  reached,  when 
the  width  of  the  pencil  is  sufficient  to  fill  the  whole  aperture  of  the  objec- 
tive. In  this  case  the  resolving  power  is  twice  as  great,  the  finest  detail 
that  the  objective  can  show  being  now  equal  to  ^  For  a  0.50 

2  X  N.A. 

objective  the  first  would  be  one  micron  with  reduced  aperture,  and  the 
second  one-half  micron  with  full  aperture. 

For  practical  purposes  we  may  take  the  rule  that  the  numerical  aperture 
multiplied  by  100,000  will  give  the  number  of  lines  per  inch  which  can  be 
resolved  theoretically  by  a  given  objective.  Thus,  pearlite  of  say  25,000 
laminations  per  inch  should  be  easily  resolved  with  an  8  millimeter  ob- 
jective of  0.50  numerical  aperture,  and  a  suitable  eyepiece  and  bellows 
draw  can  be  used  to  magnify  the  image  to  the  size  desired,  say  400  diame- 
ters. On  the  other  hand,  an  objective  of  say  0.25  numerical  aperture 
could  scarcely  be  expected  to  resolve  this  structure  no  matter  how  high  we 
magnify  the  image  by  eyepieces  or  bellows  extension.  The  magnification 
should  never  exceed  1000  times  the  numerical  aperture;  preferably  it 
should  be  somewhat  less,  dependent  upon  the  quality  of  the  objective. 

Depth  of  focus  (known  also  as  depth  of  sharpness  or  penetration)  is 
another  important  factor  which  is  often  not  clearly  understood.  It  de- 
pends on  the  numerical  aperture  and  the  magnification,  and  is  inversely 
proportional  to  both.  The  formula  used  to  express  depth  of  focus  is 

i  ;  therefore,  the  higher  the  numerical  aperture  and  the  higher  the 
N.A. 

magnification  the  less  the  depth  of  focus.  It  is  beyond  the  power  of  the 
optician  to  change  these  conditions.  Every  effort  aiming  at  an  increase  of 
the  depth  of  focus,  for  instance,  by  inserting  diaphragms  above  the  back 
lens  of  the  objective,  must  necessarily  decrease  the  effective  diameter  of 
the  back  lens  and  thus  decrease  the  numerical  aperture,  thereby  lowering 
the  efficiency  of  the  objective  as  regards  resolving  power.  It  may  be  ad- 
visable at  times  to  introduce  such  diaphragming  for  the  work  to  be  done, 
and  it  will  be  shown  later  how  such  a  change  can  be  made  by  proper  use  of 
the  illuminating  system.  However,  objectives  that  show  greater  depth  of 
focus  than  others  of  the  same  numerical  aperture  are  not  well  corrected  for 
other  qualities. 

4 


The  illuminating  power  of  an  objective  is  equal  to  the  square  of  the 
numerical  aperture,  and  objectives  can  be  so  compared;  that  is,  a  lens  of 
0.25  numerical  aperture  has  an  illuminating  power  of  0.062,  while  one  of 
0.50  numerical  aperture  has  0.25  or  4  times  as  great,  provided  it  is  used  at 
the  same  magnification.  "Flatness  of  field  as  such,"  said  the  late  Dr. 
Carpenter,  "is  an  optical  impossibility."  But  by  using  compensation  or 
hyperplane  eyepieces  to  correct  the  margin  of  field,  better  effects  aye  ob- 
tained, and  by  increasing  the  depth  of  focus  by  reducing  the  used  aper- 
ture, apparent  flatness  is  secured  but  at  the  expense  of  resolving  power,  as 
stated  before.  As  an  illustration  of  this  we  refer  again  to  the  statement 
concerning  the  depth  of  focus,  which  is  equal  to  i  ,  hence,  for  an 

N.A. 

objective  of  0.50  numerical  aperture,  the  depth  would  be  2,  while  for  an 
objective  of  0.85  numerical  aperture,  the  depth  would  be  1.17. 

As  depth  of  focus  also  varies  with  the  magnification,  if  we  use  the  above 
lenses  at  the  same  magnification  we  should  naturally  have  greater  depth 
in  the  objective  of  0.50  numerical  aperture,  and  if  this  gives  sufficient  re- 
solving power  for  the  work  in  hand  it  may  be  used  to  obtain  a  flatter  field. 

Defining  power  depends  upon  the  finest  correction  of  spherical  abbera- 
tion  and  chromatic  aberration,  the  perfect  centering  of  the  lenses  of  the 
objective,  and  in  fact  on  the  general  excellence  of  the  mechanical  work  in- 
volved in  the  making,  and  it  is  here  that  the  optician  must  show  his  skill. 
It  should  be  borne  in  mind  that  the  finer  the  definition  of  an  objective, 
the  more  sensitive  it  is  to  incorrect  focusing  and  to  slight  changes  of  the 
adjustment  through  vibration,  etc.  In  constructing  objectives  their 
formulae  should  be  based  upon  rigorous  computations,  all  elements  of 
construction,  such  as  glass,  radii,  thickness  of  lenses,  separation,  etc., 
being  determined  in  advance  wholly  without  recourse  to  experiment. 
This  method  of  construction  is  the  only  one  insuring  uniformity  in  optical 
systems  of  such  intricacy  as  a  microscope  objective. 

It  will  be  well  to  consider  at  this  time  the  effect  of  tube  length,  that  is, 
the  distance  between  the  objective  and  the  eyepiece  with  which  it  is  used. 
Most  of  the  objectives  used  on  biological  and  smaller  metallurgical 
microscopes  are  corrected  for  a  mechanical  tube  length  of  160  to  170 
millimeters.  By  mechanical  tube  length  is  meant  the  distance  from  the 
shoulder  against  which  the  objective  screws  to  the  shoulder  against  which 
the  eyepiece  rests.  In  the  case  of  the  larger  metallurgical  stands  of  the 
inverted  type  this  distance  is  usually  longer,  owing  to  reflections  neces- 
sary to  secure  an  inverted  stage  and  a  convenient  position  for  the  viewing 
tube.  Therefore,  the  objectives  used  on  this  type  of  microscope  should  be 
corrected  for  this  extra  length.  The  use  of  objectives  at  incorrect  tube 
length,  especially  in  powers  over  100  diameters,  will  result  in  inferior 
definition.  This  is  especially  true  in  metallographic  work. 

Objectives  for  metallurgical  work  must  also  be  corrected  for  use  without 
coverglass,  as  specimens  of  metal  are  examined  without  covering. 

All  objectives  used  in  biological  microscopes  are  corrected  for  the  re- 
fraction which  takes  place  when  the  ray  from  the  object  passes  through  a 

5 


Fig.  5 — Effect  of  coverglass  upon  focus. 

thin  piece  of  glass.  These  objectives  are  not  suited  for  use  on  uncovered 
objects,  and  if  used  in  metallurgical  work  only  inferior  results  will  be  ob- 
tained except  with  the  very  lowest  powers,  such  as  16  millimeters  or  lower. 
To  show  the  effect  of  cover-glass,  Figure  5  has  been  taken  from  Carpen- 
ter's book  on  the  microscope.  It  will  be  seen  that  without  the  cover-glass 
an  ordinary  microscope  objective  would  focus  at  different  planes  for  the 
central  and  marginal  zones,  as  shown  at  points  Y  and  X,  but  with  the  re- 
fraction of  the  cover-glass  the  rays  all  focus  at  the  point  O. 


Aplanatic  system.  Under-corrected  system. 

Fig.  6 — Underc orrected  system  obtained  without  use  of  coverglass. 

Therefore  if  the  ordinary  objective  is  used  without  cover-glass,  it  will 
work  as  an  under-corrected  system,  as  shown  in  Figure  6,  whereas  when 
corrected  for  use  without  cover  the  objective  should  be  perfectly  aplana- 
tic,  as  shown  at  the  left,  all  rays  focusing  at  one  point.  This  correction 
cannot  always  be  accomplished  by  changing  distances  between  lenses  but 
requires  special  computation.  The  oil-immersion  objective,  however, 
needs  no  special  correction  as  regards  cover-glasses,  because  the  immer- 
sion oil  and  cover-glass  are  of  the  same  refractive  index,  and  if  the  cover  is 
missing,  the  additional  strata  is  made  up  by  a  thicker  layer  of  oil.  The 
tube  length,  however,  must  be  correct  even  in  this  case. 

As  to  the  objective  mounts,  both  long  and  short  mounts  are  used  in 
metallurgical  work.  The  short  mounts  should  always  be  used  with  mirror 
or  prism  illuminators  so  as  to  bring  the  illuminator  as  near  to  the  back 
focus  of  the  objective  as  possible.  The  tendency  of  late,  however,  has 
been  to  use  short  mounts  for  all  kinds  of  illuminators. 

The  objectives  previously  described  are  of  the  achromatic  type,  but  the 
same  characteristics  are  also  found  in  the  apochromatic  objectives,  and 
they  are  even  more  sensitive  to  deviations  from  the  standards  for  which 
they  are  made.  The  usual  focal  lengths  and  numerical  apertures  are  as 
follows : 


Focal  Length 

1 6  millimeters 
8 

4  u 

3 

2 


Numerical  Aperture  (N.A.} 
apochromatic       achromatic 

0.30 

0.65 

-0-95 
0.95 


0.25 
0.50 
0.85 
0.85 
1.25 


It  will  be  seen  that  the  numerical  apertures  are  somewhat  greater  in  the 
apochromatic  objective.  This  means  increased  resolving  power  and  other 
features  attending  higher  aperture.  These  lenses  have  other  improve- 
ments than  increased  aperture. 

The  word  apochromatic  means  free  of  color  and  is  illustrated  by  Figure 
7.  If  we  were  to  form  an  image  with  an  ordinary  lens  of  non-achromatic 
form,  a  rayof  white  light  coming  from  the  object  would  be  broken  up  into 
the  several  colors  of  the  spectrum  and  images  would  be  formed  at  differ- 
ent distances  from  the  lens,  corresponding  to  each  color.  There  would 
thus  be  a  series  of  images,  one  behind  the  other,  ranging  from  violet  to  red, 
the  shorter  waves  being  refracted  more  than  the  longer  ones. 


Fig.   / — Illustration   of  apochromatic  lens.     Fig.  8 — Correction  for  spherical  aberration. 


In  the  standard  achromatic  objective,  two  of  these  colored  images 
would  be  combined,  and  in  apochromatic  objectives  three  colored  images 
would  form  in  the  same  plane,  and  as  the  violet  rays  are  brought  to  a  focus 
at  the  same  plane  as  the  applegreen  visual  rays,  these  objectives  are  ex- 
cellent for  photographic  purposes,  even  when  used  without  filters.  As 
shown  by  Figure  8,  the  correction  for  spherical  aberration  is  more  perfect. 
Spherical  aberration  refers  to  the  fact  that  in  an  ordinary  uncorrected  lens 
the  portion  of  a  ray  coming  through  the  center  and  the  portion  coming 
through  the  outer  zones  are  focused  at  different  planes.  In  the  uncorrect- 
ed lens  this  applies  to  all  colors;  in  the  achromatic  objective  the  correction 
is  made  for  one  selected  color,  say  green,  and  in  the  apochromatic  for  two 
colors.  Therefore,  for  the  very  finest  class  of  investigations  the  apochro- 
matic objectives  are  essential  owing  to  their  finer  color  and  spherical  cor- 
rections. These  corrections  are  accomplished  by  the  use  of  special  glasses 


and  a  transparent  mineral  substance  known  as  fluorite.  Another  series 
of  objectives  now  offered  upon  the  market  is  the  4  millimeter  dry  and  1.9 
millimeter  oil  immersions,  known  as  semi-apochromatic  objectives.  They 
have  one  fluorite  element  and  are  much  better  than  the  regular  achromatic 
objectives.  They  make  a  good  compromise  where  cost  will  not  permit  the 
purchase  of  apochromatics. 

EYEPIECES 

The  second  part  of  the  optical  equipment  to  be  considered  is  the  eye- 
piece, so  called  because  it  is  used  near  the  eye.  This  may,  however,  lead  to 
some  confusion,  as  the  eyepiece  is  also  necessary  when  forming  the  image 
upon  the  ground  glass  of  the  camera.  Some  users  of  this  class  of  apparatus 
have  been  found  who  thought  the  eyepiece  was  needed  only  in  visual 
work.  Small  photographic  lenses  are  used  without  eyepieces,  as  later  ex- 
plained. The  ordinary  eyepiece  shown  in  Figure  9  is  known  as  the 


Fig.  p — Eyepiece  of  Huygenian  form. 


POSITIVE  COMPENSATION  OCULAR 
(From  Spitta,  p  no). 

Fig.  10 — Compensation  type  eyepiece. 


Huygenian  form  after  its  designer.  It  consists  of  the  two  non-achromatic 
plano-convex  lenses  with  a  diaphragm  for  limiting  the  field  between  them. 
The  upper  lens  is  known  as  the  eye  lens  and  the  lower  as  the  field  lens. 
Huygenian  eyepieces  are  made  in  a  variety  of  powers,  as  5  X,  6.4  X,  7.5  X, 
and  loX  and  12. 5  X,  these  designations  meaning  that  they  magnify  the 
image  formed  by  the  objective  by  these  amounts.  Different  makers  used 
different  designations  for  their  eyepieces  and  this  is  likely  to  cause  confu- 
sion if  attempt  is  made  at  comparison. 

In  the  case  of  Leitz  and  Zeiss  the  eyepieces  are  numbered  from  o  to  5, 
but  the  eyepieces  of  the  Bausch  &  Lomb  Optical  Co.  are  marked  with  their 
magnifying  power  as  5  X,  loX,  etc. 

A  new  eyepiece  known  as  the  hyperplane  produces  a  flatter  image  plane 
than  the  Huygenian  eyepiece  and  allows  for  a  larger  field  of  view,  well 


adapted  for  photomicrography.  These  eyepieces  have  a  compensation 
about  half-way  between  the  Huygenian  and  compensation  eyepieces  de- 
scribed below. 

The  hyperplane  eyepieces  are  made  in  powers  from  5  X  to  20  X  and  may 
be  used  with  all  classes  of  objectives. 

Another  form  of  eyepiece  is  known  as  the  compensation-  eyepiece, 
shown  in  Figure  10,  so  called  because  it  compensates  for  the  variation  in 
size  of  the  blue  and  red  images  given  by  the  apochromatic  objective.  While 
the  apochromatic  objective  is  so  constructed  as  to  bring  these  colors  into 
focus  at  the  same  plane  as  previously  stated,  the  images  formed  are  not  of 
the  same  size,  and  this  variation  is  neutralized  by  the  compensation  eye- 
piece. The  latter  should,  therefore,  always  be  used  when  one  is  using 
apochromatic  objectives,  and  may  also  be  used  to  good  advantage  with 
the  high-power  achromatic  objectives.  The  compensation  eyepieces  of 
various  makers  are  not  always  interchangeable  with  different  apochro- 
matic objectives. 

These  eyepieces  vary  in  construction  and  in  the  number  of  lenses  ac- 
cording to  their  power.  They  are  made  in  powers  from  5  X  to  25  X,  much 
higher  than  the  Huygenian  type,  and  it  is  possible  to  use  these  high  powers 
with  the  apochromatic  objectives  owing  to  their  finer  color  correction. 

There  is  another  type  of  eyepiece  known  as  the  projection  eyepiece,  in 
which  the  eye  lens  or  lens  nearer  to  the  photographic  plate  is  adjustable 
according  to  the  bellows  extension  of  the  camera.  With  this  adjustment 
one  is  able  to  use  the  objective  in  the  same  relative  position  to  the  object 
as  it  is  when  used  for  visual  work.  Some  of  these  eyepieces  have  a  very 
small  opening  in  the  diaphragm,  which  is  likely  to  indicate  that  the  field 
is  unusually  flat. 

It  is  possible  to  get  effects  similar  to  those  produced  by  the  projection 
eyepieces,  with  ordinary  eyepieces,  by  partially  withdrawing  them  from 
the  eyepiece  tube  when  making  photographs.  This  distance  may  be 
found  by  the  formula: 

square  of  eyepiece  focus 
length  of  camera  draw 

Thus  for  a  5  X  eyepiece  of  50  millimeter  focus  and  a  bellows  draw  of  250 
millimeters,  the  eyepiece  would  have  to  be  extended  10  millimeters,  or  it 
may  be  determined  by  experiment.  It  will  be  seen  that  the  distance  be- 
comes greater  with  low-power  eyepieces  and  short  bellows  draw.  This 
adjustment  keeps  the  objective  in  the  same  relation  to  the  object  as  when 
used  in  visual  work,  as  previously  stated,  and  preserves  its  best  spherical 
correction,  tending  to  flatten  the  fields. 

Too  much  stress  cannot  be  laid  upon  the  necessity  of  keeping  all  of  the 
optical  parts  clean  and  free  from  dust,  grease,  or  finger  marks.  Poor  re- 
sults are  often  due  to  this  neglect. 


VERTICAL  ILLUMINATOR 

In  Carpenter's  book  on  the  microscope  it  is  stated  that  the  first  vertical 
illuminator  was  made  and  used  by  Prof.  H.  L.  Smith  of  Geneva,  New 
York.  Smith's  illuminator  consisted  of  a  piece  of  speculum  metal  so 
placed  as  to  reflect  light  down  through  one  side  of  the  objective,  the  image 
returning  through  the  opposite  side.  The  plane-glass  illuminator  is  said  to 
have  been  designed  by  Beck  of  England.  In  this  form  the  light  is  re- 
flected by  the  surface  of  the  glass,  through  the  objective  and  the  image  re- 
turns from  the  specimen  through  the  glass  to  the  eyepiece.  Formerly 
much  flare  was  experienced  in  using  this  type  of  illuminator,  but  it  can  be 
eliminated  by  proper  illuminating  appliances  and  diaphragms.  Both 
forms  of  illuminator  are  in  use  today,  the  first  in  the  form  of  mirrors  and 
the  latter  much  in  its  original  form. 

The  mirror  and  prism  types  are  preferred  by  some  workers,  and  while 
they  are  satisfactory 'for  low  powers  and  relief  work,  they  reduce  the 
available  aperture  of  the  objective  by  nearly  one-half  with  consequent 
reduction  in  resolving  power.  They  do  give  increased  illumination,  due 
to  total  reflection,  and  may  be  used  for  projecting  metal  specimens  with 
low  powers  when  several  observers  wish  to  study  the  image.  For  high 
powers  and  utmost  resolution,  the  plane-glass  reflector  is  best. 

Vertical  illuminators  have  appeared  from  time  to  time  in  many  forms. 
The  one  shown  in  Figure  1 1  is  a  base  model  to  which  can  be  fitted  mirrors, 
reflector,  plane-glass  reflector,  lenses  and  diaphragms,  as  desired.  For 
visual  work,  side  tubes  with  lenses  and  lamps  may  be  attached  and  used 
successfully,  but  when  high-power  illumination  is  used  and  the  beam  of 
light  must  be  carefully  centered  to  the  objective,  the  best  results  can  be  ob- 
tained by  having  condensers  and  lamps  on  separate  standards.  A  recent 
addition  to  this  type  of  illuminator  consists  in  clamping  screws,  whereby 
all  adjustments  can  be  locked  so  that  once  adjusted  the  illuminator  cannot 
be  deranged  easily.  Care  must  be  exercised  to  keep  the  glasses  of  the  il- 


Fig.  if — Vertical  illuminator  to  which  can  be 

attached  mirrors^  reflector^  plane  glass  reflec-     Fig.  12 — Condenser    and    iris    diaphragm 

tor,    lenses    and    diaphragms    as    desired.  mounted  on  a  separate  standard. 

10 


luminator  clean  and  to  prevent  distortion  by  bending  the  mount  when 
cleaning  as  it  will  tend  to  destroy  the  definition. 

Figure  12  shows  a  condenser  and  iris  diaphragm  mounted  on  a  separate 
standard  in  place  of  mounting  it  integral  with  the  vertical  illuminator. 
This  is  termed  by  the  makers  a  supplementary  condenser,  to  distinguish  it 
from  the  condenser  at  the  arc  lamp.  Its  use  and  adjustment  will  be  ex- 
plained later. 


Fig.  /j — Method  for  obtaining  critical  illumination. 

Critical  Illumination:  It  is  generally  agreed  by  expert  microscopists 
that  the  best  results  are  obtained  when  the  specimen  is  examined  or  photo- 
graphed with  critical  illumination.  Critical  illumination  is  obtained  when 
the  image  of  the  illuminant  and  the  surface  of  the  specimen  are  about  in 
one  plane,  also,  when  removing  the  eyepiece  and  examining  the  back  lens 
of  the  objective  through  a  pinhole  cap,  it  is  possible  entirely  to  fill  the  lens 
with  light.  In  Figure  13  is  shown  how  this  kind  of  illumination  may  be  ob- 
tained. The  arc  or  filament  is  nearly  imaged  on  the  supplementary  con- 
denser No.  2  by  the  aspheric  condenser  No.  i  with  a  distance  of  about  2< 
to  30  inches  between  the  two  lenses.  This  should  give  an  image  of  the  il- 
luminant of  about  25  to  30  millimeters  diameter.  The  image  should  be 
slightly  ahead  of  the  supplementary  lens  No.  2  if  an  arc  is  used  so  as  to 
avoid  imaging  gas  bubbles  upon  the  specimen. 

An  image  of  the  fully  and  evenly  illuminated  aspherical  lens  No.  i,  and 
its  iris  diaphragm,  referred  to  later,  are  projected  into  the  back  focus  of 
the  objective  by  means  of  the  supplementary  condenser  No.  2  and  the 
vertical  illuminator.  The  supplementary  lens  is  of  such  focus  and  set  at 
such  distance  from  the  objective,  (that  is,  the  same  optical  distance  as  the 
eyepiece  diaphragm  from  objective)  that  the  image  of  the  arc,  the  evenly 
illuminated  lens  No.  2  and  its  iris,  are  imaged  by  the  objective  upon  the 
specimen  when  the  objective  is  at  proper  focus.  By  closing  the  iris  in 
front  of  the  aspheric  condenser,  the  aperture  of  the  light  cone  entering  the 
objective  is  reduced,  similarly  to  that  accomplished  on  some  outfits  by 
placing  an  iris  diaphragm  on  the  side  of  the  vertical  illuminator.  This  re- 
duction of  the  light  cone  below  the  aperture  of  the  objective  gives  de- 
creased illumination,  and  decreases  the  numerical  aperture  with  attending 
results;  for  example,  less  resolving  power,  greater  depth  of  focus,  and 
greater  flatness.  Full  apertures  are  obtained  with  iris  opening  of  16 
millimeters  for  4  millimeter  objectives,  and  18  millimeters  for  16  and  32 
millimeter  objectives. 

11 


Fig.  14 — Tassln  microscope  with  illum- 
inating device  attached  to  vertical  illum- 
inator. 


Closing  the  iris  of  the  supple- 
mentary lens  No.  2,  when  imaged 
on  the  specimen,  reduces  the  size 
of  the  illuminated  spot  upon  the 
specimen,  and  hence  the  size  of  the 
visible  field.  By  just  opening  this 
diaphragm  enough  to  clear  the 
field  of  view,  unnecessary  rays  are 
cut  off,  so  that  they  will  not  cause 
reflections  on  the  tubes  of  the  ob- 
jective, illuminator,  etc. 

The  light  returning  from  the 
specimen  after  passing  through  the 
objective  and  vertical  illuminator, 
is  reflected  by  a  stellite  mirror  to 
the  eyepiece.  The  use  of  a  metal 
mirror  reduces  all  chance  of  flare 
which  might  come  from  the  plane 
surface  of  a  glass  prism. 

With  the  appliances  just  mentioned,  the  illumination  scheme  for 
metallurgical  work  as  described  in  the  Zeiss  circular,  Micro  No.  89,  may 
be  obtained,  although  the  adjustments  are  somewhat  different.  It  should 
be  noted  that  in  both  visual  and  photographic  work,  decrease  in  illumina- 
tion should  be  accomplished  by  filters  and  not  by  diaphragms  which 
reduce  aperture,  unless  reduction  of  aperture  is  desired  to  secure  certain 
results.  It  will  be  shown  later  how  these  optical  parts  are  adjusted  to 
secure  the  best  results  as  regards  centering. 

MICROSCOPES 

Any  ordinary  microscope  with  proper  optical  parts  may  be  used  in  the 
examination  of  metals,  but  it  is,  in  this  case,  either  necessary  to  mount 
each  and  every  specimen  at  a  fixed  height  to  avoid  readjustment  of  the 
illumination  or  to  have  an  illuminating  device  attached  to  the  vertical 
illuminator,  as  shown  in  Figure  14.  This  stand  is  somewhat  like  the  work- 
shop microscopes  of  Swift  and  Watson,  after  the  design  of  Mr.  Stead. 
The  illuminating  device  consists  of  a  6-volt  incandescent  lamp,  operating 
on  the  regular  no-volt  circuit,  and  two  condensers.  As  can  be  seen  from 
the  illustration,  it  is  attached  to  the  illuminator  and  moves  with  it  and  the 
optical  parts.  It  may  be  applied  to  any  ordinary  microscope,  and  the 
results  obtained  are  good  for  magnifications  up  to  about  200  diameters  in 
visual  work.  It  may  also  be  used  in  photography  for  about  100  magnifi- 
cations, but  of  course  cannot  compare  with  the  better  instruments  and 
cameras.  • 

The  general  form  of  stand  for  metallurgical  work,  however,  is  one  in 
which  the  stage  and  specimen  are  movable  by  rack  and  pinion,  as  by  this 
method  varying  thicknesses  of  specimen  can  be  accommodated  without 

12 


Fig.   i 5 — Typical  metallurgical  microscope 
for  student  use. 


Fig.  16 — Same  as  instrument  in  Fig.  i$  ^ut 

with  mechanical  stage  and  horizontal  camera 

tube  attached. 


deranging  the  illuminating  system.  Instruments  of  this  type  require  the 
mounting  of  the  specimen  in  wax  or  other  form  of  holder.  In  the  case  of 
steel  specimens,  a  Sauveur  magnetic  holder  may  be  used,  the  specimen 

being  held  from  the  under  side  by 
magnetism. 

Figure  15  shows  a  typical  form  of 
metallurgical  microscope  to  which  a 
mechanical  stage  may  be  attached 
if  desired.  The  stand  illustrated  in 
Figure  16  differs  from  the  preceding 
model  in  that  it  has  a  side  tube  for 
connecting  to  the  camera,  so  that 
visual  observations  can  be  made 
without  disconnecting  or  moving 
the  camera,  the  only  operation  nec- 
essary being  the  withdrawal  of  the 
side  tube,  with  its  prism,  from  the 
optical  axis  when  making  visual  ob- 
servation or  in  adjusting  the  light 
and  the  specimen. 

A  later  development  of  this  type 
of  stand  (Figure  17)  provides  more 
space  between  the  stage'and  objec- 


Fig.  // — Late  model  metallurgical  microscope, 
with  unusual  range  for  height  of  specimen. 

13 


tive,  an  improved  type  of  vertical 
illuminator  and  a  series  of  adjust- 


ments  and  attachments  which 
makes  it  an  almost  universal  in- 
strument for  general  laboratory 
purposes.  It  may  be  quickly  con- 
verted from  vertical  to  oblique 
or  substage  illumination,  may  be 
fitted  with  polarizer  and  analyzer 
for  either  opaque  or  transparent 
specimens,  and  is  adaptable 
through  the  use  of  suitable  at- 
tachments for  either  binocular  or 
monocular  wide  field  work  at 
comparatively  low  powers.  It  is 
particularly  adapted  for  metal- 
lographic  work  in  connection  with 
any  of  the  standard  photomicro- 
graphic  cameras  as  the  stage  ad- 
justment permits  focusing  with- 
out disarranging  the  alignment 
of  the  illuminating  equipment. 

Many  workers  prefer  a  stand 
of  the  inverted  type,  that  is,  one 
in  which  the  stage  is  above  in- 
stead of  below  the  objective,  and 
in  which  it  is  necessary  to  prepare 
one  surface  of  the  specimen  only, 
the  polished  face  being  placed, 
face  down  upon  the  stage  over 
the  objective.  The  inverted  form 
of  microscope  is  credited  to  Le 
Chatelier  in  the  year  1897. 

It  has  been  made  in  a  variety 
of  forms,  first  by  Pellin  of  Paris 
(Fig.  1 8),  later  by  Dujardin  & 
Company  (Fig.  19).  Other  models 
have  been  made  by  Leitz  (Fig. 
20)  of  Germany  and  Reichert  of 
Austria. 

*Figure  21  shows  the  latest 
large  inverted  microscope  of  the 
Bausch  &  Lomb  Optical  Com- 
pany, which  embodies  all  the  de- 
sirable features  of  a  metallur- 
gical microscope.  This  instru- 
ment has  been  so  designed  that 
relative  vibration  between  the 
specimen  and  the  objective  can 


a 

3D 

1 


I 


Fig.  20 — Inverted  metallurgical  microscope.   E.  Leitz. 


not  exist.  Furthermore,  the  fine  adjustment  has  been  mounted  so  that  it 
is  required  to  move  a  weight  of  only  four  ounces.  When  an  illuminant 
of  high  intensity  is  used,  considerable  heat  is  bound  to  be  received  by  the 
vertical  illuminator.  For  this  reason  the  objective  is  mounted  independent 
of  the  vertical  illuminator  and  whatever  heat  reaches  the  latter  and  causes 
it  to  expand  will  not  change  the  distance  between  the  specimen  and  ob- 
jective and  consequently  will  not  cause  the  image  to  go  out  of  focus.  A 
heat  guard  is  provided  to  prevent  any  light  other  than  the  centered  beam 
from  reaching  any  part  of  the  microscope. 

The  body  of  the  microscope  consists  of  a  casting  in  which  is  mounted  a 
stellite  mirror  which  reflects  the  image  received  from  the  objective  to- 
wards the  photographic  plate.  Over  this  stellite  mirror  is  mounted  the 
vertical  illuminator.  The  body,  including  the  eyepiece,  stellite  mirror 
and  vertical  illuminator  are  all  in  one  rigid  mounting.  The  pillar  on  which 
this  body  is  mounted  is  short  and  sturdy.  It  extends  vertically  downward 
to  the  base  and  carries  on  it  a  rack  slide. 

The  entire  stage  enclosing  the  microscope  body  is  a  single  casting.  At- 
tached to  the  bottom  of  the  casting  is  a  rack  and  slide  which 'moves  on  the 
pillar.  Vertical  motion  is  accomplished  by  means  of  the  rack  and  pinion. 
The  inner  edges  of  the  two  stage  posts  nearest  the  camera  are  accurately 
milled,  forming  a  double  slide.  A  pair  of  pins,  mounted  in  an  upright 
which  is  securely  fastened  to  the  top  of  the  body>  maintain  a  constant 
pressure  against  these  milled  edges.  This  even  tension  on  both  sides  of  the 
stage  posts,  very  near  the  surface  of  the  specimen,  assures  perfect  stability. 
(Figure  22.) 

On  the  microscope  .pillar,  opposite  the  stellite  mirror  position,  is  a 
strong,  broad  fine  adjustment  slide  on  which  the  objective  support  moves. 
(Figure  23).  At  the  bottom  of  this  slide  is  the  fine  adjusting  mechanism 
which  is  required  to  raise  and  lower  the  objective  and  its  mount  only. 
The  support  for  the  objective  is  only  slightly  below  the  surface  of  the 
stage,  and  the  fulcrum  about  which  vibrations  would  take  place  is  in  the 
plane  of  the  objective  shoulder.  This  shoulder  is  level  with  the  pair  of  pins 
which  steady  the  stage. 

16 


18 


*Fig.  21 — Eausch  &?  Lomb  Metallurgical  Microscope. 


1.  Specimen  Holder 

2.  Stage  Aperture  Plate 

3.  Mechanical  Stage  Scale 

4.  Mechanical  Stage  Adjustment 

Heads. 

5.  Objective 

6.  Objective  Handle 

7.  Iris  Diaphragm  Adjusting  Ring 


8.  Filter  Mount 

9.  Vertical  Illuminator  Mirror 

Mount 

10.  Stellite  Mirror  Housing 

11.  Heat  Shield  Socket 

12.  Microscope  Body 

13.  Observation  Eyepiece 

14.  Camera  Connector 


15.  Stage  Casting 

1 6.  Coarse   Adjustment    Head 

17.  Fine  Adjustment  Head 

1 8.  Reducing  Gear  Lever 

19.  Coarse  Adjustment  Scale 

20.  Coarse  Adjustment  Lock 

21.  Stabilizer 


17 


The  screw  that  actuates  the  Bj 
fine  adjustment  produces  a  ver- 
tical movement  of  0.125  mm  per 
revolution.  A  large  diameter 
button  may  be  brought  into  con- 
tact with  a  reducing  speed  shaft 
by  moving  a  cam.  This  provides 
a  more  sensitive  fine  adjustment 
for  use  in  focusing  the  image  from 
the  ground  glass  position.  One 
revolution  of  the  rod  in  focusing 
on  the  ground  glass  produces  a 
vertical  movement  of  the  object- 
ive of  only  0.03  mm.  The  fine 
adjusting  head  is  graduated  to 
read  to  2.5  microns  of  vertical 


*Fig.  22 — Lateral  stage  support  stabilizer. 


movement  but  may  be  estimated  much  closer. 

The  coarse  adjustment  consists  of  a  broad,  heavy  slide,  on  which  the 
stage  casting  moves,  actuated  by  a  heavy  rack  and  pinion  controlled  by 
knurled  head.  The  position  of  the  slide  may  be  fixed  definitely  and 

securely  after  an  approximate  focus 
is  obtained  by  means  of  a  lock  actu- 
i  ated  by  a  convenient  screw.  This 
support  and  lock  at  the  bottom,  and 
the  stabilizer  at  the  top  of  stage  sup- 
port, as  described  above,  effectually 
prevent  any  possibility  of  vibration. 
The  coarse  adjustment  operates 
by  moving  the  stage  and  specimen 
with  respect  to  the  objective.  A  con- 
venient scale  and  index  permits  the 
specimen  to  be  brought  into  ap- 
proximate focus  for  the  objective 
being  used. 

The  fine  adjustment  (Figure  23) 
consists  of  a  broad  dovetail  slide, 
actuated  by  a  knurled  head  and 
lever  action,  the  same  as  on  the 
finest  biological  microscopes.  The 
slide  is  mounted  on  the  pillar  of  the 
microscope  so  that  it  is  movable 
with  respect  to  the  vertical  illumin- 
ator. The  slide  is  carried  on  a  sturdy 
vertical  rod,  at  the  lower  extremity 
of  which  is  the  fine  adjustment  mech- 
anism and  at  the  top  a  horizontal 
table  on  which  the  objective  holder 


*Fig.  23 — Fine  adjusting  mechanism  (Phantom 


view). 


18 


is  mounted.  The  latter  is  so  designed  that  the  objectives  are  easily  inter- 
changeable and  are  held  against  a  shoulder  and  stop  in  such  a  way  that  they 
are  positively  centered  with  respect  to  the  optical  axis  of  the  instrument. 

This  construction  has  a  number  of  advantages  over  any  previous  de- 
signs of  any  manufacture.  The  most  important,  of  course,  is  the  fact 
that  the  fine  adjustment  carries  the  smallest  possible  weight  so  that  re- 
gardless of  the  weight  of  the  specimen  on  the  stage  there  is  no  additional 
strain  on  the  mechanism.  In  other  constructions  this  mechanism  carries 
so  much  weight  that,  where  long  exposures  are  necessary,  it  is  often  im- 
possible to  prevent  the  microscope  slipping  or  settling  out  of  focus  during 
the  exposure. 

Another  cause  of  change  of  focus  in  previous  constructions  is  the  ef- 
fect of  heat  from  the  illuminant  on  the  metal  parts  of  the  apparatus.  The 
beam  of  light  first  enters  the  vertical  illuminator  through  the  filter  and 
condenser,  which  absorb  much  of  the  heat  and  transmit  some  of  it  to  their 
respective  mountings  which  expand  as  a  result.  Where  the  objective  is  car- 
ried on  the  same  fixture  as  the  other  optical  parts,  this  expansion  is  bound 
to  displace  the  objective  with  respect  to  the  specimen  so  as  to  destroy  the 
sharp  definition  at  the  plate.  In  this  new  construction,  the  objective 
being  mounted  independently  of  the  vertical  illuminator,  no  effect  of  the 
expansion  of  the  parts  is  transmitted  to  the  focusing  adjustment. 

A  telescoping  tube  connector  between  the  objective  and  the  vertical 
illuminator  excludes  extraneous  light  and  reflections  from  this  part  of  the 
optical  system.* 


Fig.  24 — Marten's  metallurgical  microscope.   (Zeiss) 
19 


Fig.  25 — Rosenhain  metallurgical  microscope.  R.  &  J.  Beck. 

Other  types  of  metallurgical  microscopes  which  should  be  mentioned 
are  those  of  Martens,  made  by  Zeiss,  (Fig.  24),  and  the  Rosenhain  micro- 
scope, made  by  Beck  (Fig.  25).  The  Martens'  stand  is  of  horizontal  type, 
provided  with  movable  stage,  and  is  in  general  form  similar  to  the  stands 
first  described.  The  specimen  must  be  mounted  so  as  to  retain  the  pol- 
ished surface  in  a  vertical  position.  The  illuminating  apparatus  is 
placed  at  right  angles  to  the  axis  of  the  microscope.  The  Rosenhain  is  an 
enlarged  and  heavy  form  of  usual  type  and  has  its  vertical  illuminator 
built  into  the  tube. 

20 


ILLUMINATING   SYSTEMS 


Fig.  26 — Arc  lamp.  Hand  feed  type.  *Fig.  27 — Arc  lamp.    Automatic  feed  type. 

The  direct  current  arc  of  low  amperage  is  no  doubt  the  best  illuminant 
to  use  if  intensity  is  desired.  The  lamp  may  be  of  the  hand-feed  variety, 
but  it  is  better  to  use  a  lamp  having  an  automatic  mechanism  for  feeding 
the  carbons.  Figure  26  shows  a  lamp  of  hand-feed  type,  and  Figure  27 
shows  the  automatic  feed  type,  both  lamps  interchanging  on  the  same 
standard.  Suitable  condensers  must  be  used  to  collect  a  wide  angle  of  the 
beam  emitted  from  the  lamp  and  condense  it  into  the  vertical  illuminator. 
Centering  screws  are  usually  provided  so  that  the  illuminant  can  be  well 
centered  to  the  condensing  lenses. 


*Fig.  28 — Illuminator  and  microscope  mounted  as  one  unit  with 
permanently  aligned  optical  system. 

21 


The  carbons  used  on  direct  current  for  4  to  5  amperes  are  usually  8 
millimeters  diameter  for  the  positive  or  horizontal  carbon  and  6  milli- 
meters for  the  negative.  It  is  necessary  to  see  that  the  positive  wire  of  the 
electric  current  is  on  the  horizontal  carbon.  This  can  be  determined  by 
noting  the  brightness  of  the  carbons  through  the  smoked-glass  window, 
the  brighter  one  being  the  positive. 

For  alternating  current  the  carbons  should  be  7  millimeters  diameter 
for  both  horizontal  and  vertical,  and  it  makes  no  difference  how  the  wires 
are  connected. 

For  visual  observation  and  for  photography  where  speed  is  not  essen- 
tial, a  Mazda  lamp  with  ribbon  form  of  filament  makes  a  very  acceptable 
illuminant.  It  has  the  advantage  of  giving  a  constant  and  uniform  il- 
lumination and  requiring  no  care  when  once  adjusted.  This  lamp,  how- 
ever, uses  only  6  volts  and  is  best  used  with  an  alternating  current  supply 
in  connection  with  a  transformer,  as  the  amperage  rating  is  1 8. 

In  earlier  models  it  was  customary  to  have  all  of  the  different  parts  ad- 
justable relative  to  each  other  on  the  optical  bed  and  in  both  vertical  and 
lateral  directions.  This  was  done  so  that  the  microscope,  illuminating 
system  and  camera  could  be  all  aligned  by  the  operator.  For  the  operator 
who  was  trained  in  the  manipulation  of  complicated  optical  apparatus 
this  construction  presented  few  difficulties  of  technique  but  did  consume 
a  great  deal  of  time  in  the  preliminary  setting  up  of  the  apparatus  every 
time  work  was  to  be  done. 

*Experience  shows  that  there  is  no  particular  advantage  in  this  con- 
struction. The  experienced  workman  at  the  factory,  by  means  of  col- 
limators  and  gauges  can  easily  line  up  the  illuminating  and  optical  systems 
so  that  they  will  work  together  at  their  highest  possible  efficiency  and  then 
lock  them  securely  in  position.  After  this,  no  further  adjustment  is  neces- 
sary except  to  bring  the  light  source  into  center.  The  most  critical 
technician  can  ask  nothing  more  than  a  perfectly  centered  beam  from  the 
illuminant  into  the  vertical  illuminator  regardless  of  the  kind  of  illumina- 
tion used  on  the  specimen.  This  is  the  principle  on  which  the  Bausch  & 
Lomb  Large  Metallographic  Equipment  is  constructed  and  which  is  called 
"permanently  aligned."  (Figure  28.) 

In  the  new  instrument  all  accessories  such  as  the  liquid  cell,  filter  holder, 
iris  diaphragm  and  decentering  device  for  oblique  illumination  are  con- 
veniently mounted  between  the  light  source  and  microscope  and  no  ad- 
justment of  any  of  these  parts  can  throw  the  beam  out  of  its  true  center. 

We  believe,  and  experience  has  shown,  that  this  construction  is  in  every 
way  the  most  satisfactory  forward  step  taken  in  the  photomicrography  of 
metallurgical  specimens  since  Le  Chatelier  introduced  the  inverted  micro- 
scope, because  it  permits  anyone  to  obtain  satisfactory  results  regardless 
of  their  optical  training  or  skill. 

The  mounting  of  the  vertical  illuminator  on  the  microscope  in  a  fixed 
position  makes  it  a  part  of  the  permanently  aligned  system  of  which  men- 
tion has  been  made  above.  Being  in  fixed  position  it  is  not  thrown  out  of 
center  with  respect  to  the  axis  of  the  beam  when  the  objective  is  focused 

22 


on  the  specimen  by  means  of  the  fine  adjustment,  as  is  the  case  in  instru- 
ments in  which  the  vertical  illuminator  and  objective  are  both  carried  on 
fine  adjustment  mechanism. 

The  vertical  illuminator  consists  of  a  square  metal  box,  mounted  on  the 
upper  surface  of  the  microscope  base  and  having  three  openings.  Facing 
the  illuminant  is  a  tube  carrying  the  filter,  iris  diaphragm  and  condenser 
lens.  At  the  top  is  the  opening  for  the  objective  connector,  while  facing 
the  operator  is  an  opening  carrying  a  shaft  with  knurled  head  at  the  outer 
end,  which  controls  the  clear  glass  reflector  and  prism  for  directing  the  il- 
luminating beam  up  thru  the  objective.  Either  may  be  used,  being  inter- 
changeable by  a  simple  adjustment. 

The  filter  supplied  in  the  vertical  illuminator  mount,  produces  practic- 
ally monochromatic  light  equivalent,  to  the  E  line  of  the  mercury  spec- 
trum, which  is  the  wave  length  for  which  the  achromatic  objectives  are 
corrected.  This  wave  length  has  been  found  to  be  the  most  satisfactory  for 
all  around  metallographic  work,  particularly  for  iron  and  steel.  In  case 

other  Wratten  filters  are  to  be  used,  they 
will  be  placed  in  the  filter  mount  on  the 
front  of  the  liquid  cell  attached  to  the  illu- 
minating unit. 

The  full  numerical  aperture  of  the  ob- 
jective may  be  used,  or  the  aperture  may 
be  reduced  by  means  of  the  iris  diaphragm 
in  the  tube  directly  back  of  the  filter.  The 
maximum  resolution  with  any  objective  will 
be  obtained  when  the  full  aperture  of  the 
objective  is  used.  The  flatness  of  field  can 
be  increased,  with  a  sacrifice  of  resolution, 
by  using  a  smaller  opening.  The  ILS  Mi- 
croscope can  be  diaphragmed  down  to  give 
a  flat  field — but  in  no  photomicrographic 
outfit,  metallographic  or  otherwise,  can 
full  resolving  power  and  perfectly  flat  field 
be  had  at  the  same  time. 

The  square-,  object  stage  together  with 
the  support  at  each  corner  is  cast  in  one 
solid  piece,  to  increase  the  rigidity.  This 
casting  is  mounted  on  a  heavy  L  shaped 
part,  the  longer  side  of  which  is  the  slide 
which  moves  along  the  pillar  of  the  microscope  under  control  of  a  heavy 
rack  and  pinion,  forming  the  coarse  adjustment.  The  weight  of  the 
stage  and  specimen  is  located  directly  over  the  center  of  the  coarse  ad- 
justment slide.  (Figure  29.) 

The  slide  of  the  coarse  adjustment  is  dovetailed  in  cross  section  and  is 
accurately  milled  and  fitted.  A  clamping  device  on  the  coarse  adjustment 
head  locks  the  slide  securely  in  place  so  that  no  sagging  or  vertical  motion 

23 


*Fig.  29 — Microscope  stage.  Weight 

of  stage  and  specimen  centered  over 

coarse  adjustment  slide. 


I. 

*Fig.  jo — Mechanical  stage.    Adjustment  heads  on  both 
sides  of  stage  and  all  acting  in  same  plane. 

can  take  place  under  the  weight  of  the  specimen  during  the  longest  ex- 
posures. As  a  precaution  against  the  vibration  of  the  stage  relative  to  the 
other  parts  of  the  microscope,  there  is  provided  a  stabilizer  which  prevents 
any  lateral  movement  of  the  upper  part  of  the  stage,  so  that  the  latter  is 
supported  both  top  and  bottom.  The  stabilizer  consists  of  a  Y  shaped 
brace  attached  to  the  top  of  the  microscope.  In  horizontal  position  at  the 
top,  it  carries  two  ball  ended  shafts  which  bear  against  milled  surfaces  at 
each  side  of  the  stage  supports.  This  brace  is  practically  in  the  plane  of 
the  objective  and  effectually  prevents  any  vibration  of  the  specimen  rela- 
tive to  the  objective. 

The  mechanical  stage,  (Figure  30)  which  directly  supports  the  speci- 
men has  two  horizontal  movements  at  right  angles  to  each  other,  making 
it  possible  to  thoroughly  and  systematically  explore  the  surface  of  the 
specimen.  These  movements  are  controlled  by  a  screw  and  rack  and 
pinion  actuated  by  two  shafts  rotating  in  the  horizontal  plane.  The 
shafts  bear  knurled  heads  at  each  side  of  the  microscope  so  that  manipu- 
lation may  be  accomplished  with  either  hand  and  in  the  most  convenient 
position. 

There  is  a  \y^"  diameter  opening  in  the  center  of  the  stage  plate  which 
is  turned  with  a  shoulder  to  hold  a  metal  aperture  plate.  This  plate  has  a 
long  tapering  opening  which  permits  the  use  of  any  opening  between  4mm 
and  iimm. 

Projecting  in  the  horizontal  plane  from  the  stellite  mirror  mount  is  the 
body  tube  of  the  microscope.  This  carries  the  eyepiece  at  its  outer  ex- 
tremity, together  with  a  light  tight  connector  to  exclude  stray  light  when 
using  the  camera.  Extending  at  right  angles  from  the  body  tube  is  an 
auxiliary  tube  for  visual  observation.  In  visual  observation  a  prism 
diverts  the  emergent  beam  from  the  stellite  mirror  into  this  observation 
tube,  which  is  also  equipped  with  an  eyepiece.  When  focusing  on  the 
ground  glass  this  prism  is  withdrawn  from  the  optical  axis.  This  arrange- 

24 


ment  permits  preliminary  orientation  and  focusing  of  the  specimen  before 
the  final  focusing  on  the  ground  glass  screen  of  the  camera  is  undertaken. 
The  entire  arrangement  is  such  that  observation  in  the  auxiliary  tube, 
manipulation  of  the  mechanical  stage  and  focusing  with  the  fine  adjust- 
ment are  all  accomplished  simultaneously  in  the  most  comfortable  and 
convenient  position. 

The  illuminant  and  its  accessories  are  supported  on  a  rigid  stand  that  is 
a  unit  with  the  pillar  which  supports  the  inverted  microscope.  This 
stand  supports,  in  order,  the  illuminant  in  its  adjustable  centering  mount, 
the  condensing  lenses  in  a  spiral  focusing  mount,  the  iris  diaphragm  and  a 
water  cell  and  auxiliary  filter  mount  combined. 

The  illuminant  regularly  supplied  with  this  outfit  is  the  automatic  feed 
arc  lamp  which  must  be  used  with  a  suitable  rheostat  for  the  ordinary  1 10 
V  current.  The  clock  feed  mechanism  of  the  arc  lamp  operates  the  carbon 
holders  by  means  of  a  constant  chain  drive,  maintaining  a  steady  and 
uniform  arc  of  high  intensity.  The  excursion  of  the  carbon  holders  is 
sufficient  to  accommodate  the  entire  length  of  6"  carbons  with  one  setting, 
giving  nearly  two  hours  of  steady  light. 

Interchangeable  in  the  same  mounting  is  a  108  watt  ribbon  filament 
Mazda  lamp  in  special  housing.  This  provides  a  steady  and  uniform 
light,  that  is  always  in  alignment.  It  is  preferred  by  many  because  of  its 
convenience  and  freedom  from  attention.  As  the  intensity  of  the  light 
from  the  Mazda  is  much  less  than  that  from  the  arc  the  length  of  expo- 
sure must  necessarily  be  greater.  However,  if  the  equipment  is  to  be  used 
for  extended  periods  of  observation  only,  this  lamp  is  recommended. 

The  large  iris  diaphragm  is  mounted  in  the  optical  axis  of  the  instru- 
ment, just  in  front  of  the  condensing  lenses.  It  is  held  on  a  mount  which 
is  adjustable  for  rotation  about  the  principal  axis,  for  decentration  in  any 
direction  to  the  extent  of  one  half  the  radius  of  its  total  aperture  and  for 
total  swing-out  clear  of  the  aperture  of  the  condenser.  The  diaphragm 
will  give  any  opening  from  less  than  two  millimeters  to  over  thirty  milli- 
meters and  this  construction  permits  the  opening  to  be  centered  at  any 
point  in  the  total  cross-section  of  the  beam  from  the  illuminant.  This 
makes  possible  the  illumination  of  the  specimen  by  oblique  illumination 
from  any  direction  and  a  central  stop  may  be  readily  inserted  to  provide 
for  conical  illumination. 

The  liquid  cell  is  mounted  on  an  arm  between  the  diaphragm  and  the 
microscope.  It  consists  of  a  metal  box  with  glass  windows  in  the  light 
path.  The  glass  plates  are  burnished  in  against  rubber  washers  and  are 
liquid  and  trouble  proof.  The  cover  of  the  cell  is  so  designed  as  to  also 
form  a  limiting  aperture  over  one  of  the  glass  plates  and  a  mount  for  a 
Wratten  filter  on  the  opposite  side  of  the  cell  from  the  illuminant.  The 
cell  may  be  easily  removed  for  cleaning  or  filling.  It  may  be  used  either  as 
a  water  cell  for  protecting  the  Wratten  filters  or  as  a  liquid  filter  cell. 

In  the  photomicrography  of  transparent  specimens,  two  methods  of  il- 
lumination are  in  common  usage.  These  are  critical  illumination,  in  which 
the  image  of  the  light  source  is  focused  directly  on  the  specimen,  and 

25 


Fig.jz — A  camera  especially 
designed  for  use  with  the 
side  tube  microscope. 


Fig.  32 — Simple  type   of  vertical  camera. 


Fig.  33 — A  form  of  low  stand  with 
vertical  camera. 


26 


Fig.  34 — Small  metallographic  outfit  for  student  use. 

Koehler  illumination  in  which  some  evenly  illuminated  surface  such  as  the 
condenser  surface  is  imaged  on  the  specimen.  Where  opaque  specimens  are 
concerned  the  critical  system  of  illumination  is  preferable  owing  to  the 
higher  intensity  of  light  on  the  surface  of  the  specimen. 

In  this  equipment  the  illumination  is  critical.  The  light  source  is  first 
imaged  by  the  two  lens  condenser.  In  the  opening  of  the  vertical  illumina- 
tor, facing  the  lamp,  is  a  lens  having  a  focal  length  such  that  this  source 
image  appears  at  the  microscope  objective,  to  come  from  the  plane  of  the 
diaphragm  of  the  eyepiece.  In  other  words,  the  source  image  is  again 
imaged  by  the  lens  in  the  side  of  the  vertical  illuminator  in  a  position  con- 
jugate to  the  specimen.  Thus,  when  the  objective  receives  the  light,  it 
forms  an  image  of  the  light  source  practically  on  the  surface  of  the  speci- 
men, presenting  critical  illumination  as  described  above. 

The  image  of  the  source  on  the  specimen  varies  in  size  with  the  ob- 
jective used.  The  size  of  the  image,  or  illuminated  area  is  slightly  larger 
than  the  area  of  the  field  covered  by  the  objective  and  5  X  eyepiece.  No 
light  goes  to  waste  and  when  the  objective  is  focused  on  the  specimen  the 
light  is  also  focused  on  the  specimen.  Furthermore,  the  source  image 
formed  on  the  specimen  is  free  from  spherical  and  chromatic  aberration. 
It  is  these  reasons  that  make  the  illumination  ideal  in  metallurgical  micro- 
scopes. The  highly  corrected  objective,  with  which  the  detail  in  the  speci- 
men is  brought  out,  performs  a  double  duty;  it  is  first  a  condenser  and  then 
an  objective.* 

CAMERAS.  —  The  cameras  and  supports  available  for  this  work  proba- 
bly are  as  great  in  variety  as  the  microscopes.  A  few  types  will  be  re- 
ferred to,  all  of  which  embody  supports  to  carry  an  illumination  system  of 
the  type  described.  Figure  32  shows  a  simple  type  of  vertical  camera 
which  has  been  in  use  for  a  number  of  years.  It  differs  from  the  ordinary, 

27 


Fig.  35 — Complete  Eausch  &  Lomb  Large 
Metallographic  Equipment  with  shock 
absorbers  and  stand. 


in  that  the  camera  and  all  accessories  are  mounted  on  a  single  base  board 
to  which  they  can  be  clamped  after  adjustment.  It  is  annoying  in  this 
work  to  have  a  number  of  loose  parts  which  are  readily  disturbed.  Figure 
33  shows  a  low  form  of  stand  with  vertical  camera.  When  resting  on  the 
floor  the  ground  glass  is  at  a  convenient  height  for  observation.  The 

camera  may  be  swung  to  one 
side  when  making  adjustments  of 
the  specimen  and  illumination. 

Figure  31  shows  a  type  espe- 
cially designed,  after  suggestions 
of  Professor  Sauveur,  for  use  with 
the  side-tube  microscope.  As 
stated,  when  referring  to  the  mi- 
croscope, the  side  tube  is  with- 
drawn for  visual  observation  and 
again  pushed  into  the  path  of 
light  when  making  photographs. 
The  light-tight  connector  be- 
tween camera  and  microscope  is 
so  made  that  neither  camera  nor 
microscope  need  be  moved  in 
making  the  adjustment  from  vis- 
ual to  photographic  use.  All  the 

*Fig.36-Supplementary  plate  holder  and  view-     camera  ^  supports   described    are 
ing  device  with  viewing  tube  in  position   for      made  with  either  4  by  5  or  5  by 
observation.  j  inch  cameras. 

28 


*Fig.  38 — Attachments  for  macro-photography, 
showing  arc  lamp  tilted  for  oblique  illumination. 


.    37 — Shock    absorbers^    showing  frame, 
spring  and  adjustable  rubber  damper. 


fFig.  39 — Attachments  for  macro-photography,  show- 
ing arc  lamp  in  position  for  vertical  illumination. 


The  camera  shown  in  figure  34  is  a  small  outfit  for  student  use  and  for 
industrial  plants  where  only  a  small  amount  of  work  at  low  and  medium 
powers  is  required.  The  microscope  is  of  the  inverted  type  and  is  mount- 
ed over  the  camera.  The  specimen  is  examined  by  means  of  a  side  tube, 
which,  with  its  prism,  may  be  withdrawn  from  the  optical  axis,  permitting 
the  image  to  fall  upon  the  plate  in  the  camera.  While  the  image  may  be 
focused  upon  an  opaque  screen  in  the  camera,  it  is  possible  so  to  adjust  the 
camera  eyepiece  that  the  image  is  sharp  upon  the  plate  when  focused  for 
visual  observation.  The  illuminant  is  the  6-volt,  io8-watt  Mazda  lamp. 
The  mountings  for  lamp  and  the  condensers  are  of  much  simpler  form 
than  in  the  previous  outfits,  but  will  give  satisfactory  results  in  magnifica- 
tions from  50  to  400. 

*Figure  35  shows  the  complete  Bausch  &  Lomb  Large  Metallographic 
Equipment,  the  microscope  and  illuminating  unit  of  which  has  already 
been  described. 

The  camera  parts  consist  of  a  tapering  bellows  of  40"  extension,  with 
front  and  rear  supports  and  center  frame  to  prevent  sagging.  The  camera 
back  is  reversible  and  is  provided  with  a  hinged  door,  single  book  form  of 
plate  holder  for  8  x  10  plates  with  reducing  kits  for  smaller  sizes  down  to 
3^  x  4/4*  one  ground  glass  screen  with  clear  center  for  use  with  focusing 
glass  and  a  clear  glass  screen  with  graduated  cross  lines.  The  front  sup- 
port is  fitted  with  an  exposure  shutter  and  light  tight  connector  for  con- 
necting with  microscope.  An  extension  rod  for  moving  the  fine  adjust- 
ment while  observing  the  image  on  the  ground  glass,  and  a  focusing  glass 
are  included  in  the  camera  equipment. 

29 


The  supporting  base  is  made  of  two  parts — a  heavy,  steel  double  bar 
for  the  camera  parts  and  a  section  of  accurately  milled  optical  bed  for  the 
microscope.  They  are  carefully  joined  together  and  provided  with  low 
supporting  feet  at  either  end.  The  supports  for  the  camera,  slide  very 
easily  along  the  bar  because  of  the  long  bearing  surface  and  are  prevented 
from  tipping  by  a  tongue  running  in  a  groove  on  the  side  of  the  bar.  The 
bar  is  graduated  in  millimeters  for  the  easy  determining  of  bellows  draw. 

A  new  camera  back,  Figure  36,  has  been  developed  to  make  the  focus- 
ing of  the  image  easier  and  more  positive,  and  to  eliminate  vibration  in  the 
manipulation  of  the  dark  slide. 

It  consists  of  a  frame  interposed  between  the  rear  bellows  frame  and  the 
camera  back  proper,  in  which  there  is  mounted  a  magnifier  with  a  right 
angle  prism  which  faces  toward  the  eyepiece  of  the  microscope.  To  focus, 
one  has  but  to  look  in  the  magnifier  and  adjust  the  fine  adjustment  of  the 
microscope  until  the  image  is  in  focus  on  the  cross  hairs. 

As  these  are  located  the  same  distance  from  the  prism  as  the  plane  of 
the  plate,  the  image  must  be  in  focus  in  that  plane.  The  magnifier  is 
mounted  on  a  pivot  support  so  it  may  be  moved  across  the  entire  field. 

In  this  new  back  there  is  a  supplementary  dark  slide  which  is  raised  by 
counter  balanced  weights  running  in  cylinders  on  either  side.  With  this 
device  it  is  possible  to  withdraw  the  regular  dark  slide  from  the  plate 
holder,  permit  the  equipment  to  come  to  rest,  check  up  on  the  focus  with 
the  magnifier,  swing  it  aside,  close  the  shutter,  release  the  catch  on  the 
supplementary  dark  slide,  permitting  it  to  automatically  rise  without  any 
vibration  and  then  make  the  exposure.  This  arrangement  will  be  readily 
appreciated  by  all  photomicrographers.  It  can  be  attached  to  any  of  the 
type  G  cameras  now  in  use.  Trial  exposures  may  be  made  by  withdraw- 
ing dark  slide,  giving  the  plate  an  exposure  of  15  seconds;  replacing  the 
slide  one  inch  at  a  time,  giving  each  succeeding  inch  15  seconds,  so  that 
the  final  exposure  on  a  5-inch  plate  would  be  75  seconds,  60  seconds,  45 
seconds,  30  seconds  and  15  seconds  for  the  various  parts.  After  some  ex- 
perience it  is  easy  to  judge  the  correct  exposure  for  a  given  class  of  ob- 
jects. 

It  is  a  recognized  fact  that  the  vibrations  caused  by  machinery,  street 
traffic,  etc.,  must  be  absorbed  before  successful  photomicrographs  can  be 
taken.  The  Bausch  &  Lomb  Shock  Absorbers  (Figure  37)  consist  of  four 
unit  supports,  each  unit  of  which  is  made  up  of  a  sturdy  frame,  one  coiled 
spring,  a  sponge  rubber  damper  and  a  threaded  secondary  support  for  ad- 
justing the  pressure  on  the  rubber  damper  or  absorber. 

Figure  38  shows  an  arrangement  for  attaching  low-power  photo- 
graphic lenses,  of  the  32-,  48-,  and  72-millimeter  foci,  to  camera,  and 
special  device  for  holding  and  focusing  the  specimen.  The  focusing  is 
done  by  extension  rod  from  rear  of  camera  while  observing  the  ground 
glass.  It  is  intended  for  the  photography  of  fractures,  etc.  This  also 
shows  the  illuminant  as  used  for  oblique  illumination.  Plane  glass  holders 
are  supplied  for  vertical  illumination  with  these  low-power  lenses,  as 
shown  in  Figure  39.* 

30 


>L 


12  13  D    9 


Fig.  40 — Metallographic  outfit  with  parts  numbered  for  reference. 

ADJUSTMENTS 

Many  of  the  difficulties  in  the  use  of  metallographic  equipment  are  due 
to  improper  adjustment  of  the  illuminating  and  optical  parts.  The 
later  models  of  the  Bausch  &  Lomb  Large  Metallographic  Equipment  are 
permanently  aligned  at  the  factory  by  means  of  collimators  and  gauges  so 
that  the  greatest  possible  efficiency  is  obtained.  As  the  various  parts 
are  fixed  in  position  there  is  no  possibility  of  this  arrangement  being 
disturbed  and  no  time  is  lost  in  making  the  preliminary  adjustments  each 
time  the  apparatus  is  to  be  used. 

This  model  is,  however,  comparatively  recent  and  there  are  many 
of  the  older  models  as  well  as  those  of  other  makes  in  use,  in  which  it 
is  necessary  to  align  the  optical  parts.  For  this  reason  a  brief  description 
of  the  process  is  here  given,  the  principle  of  the  operation  being  sufficiently 


FLAM  GLASS 


MIRROR 


CORRECT  /NCOffRECT  CORRECT  INCORRECT 

Fig.  41 — Method  of  adjusting  light  spot  with        Fig.  42 — Method  of  adjusting  mirror 
plane  glass.  illuminator. 

similar  in  all  cases  to  make  these  directions  applicable.     Number  refer- 
ences are  to  parts  indicated  in  Figure  40. 

One  of  the  main  points  is  the  centering  and  focusing  of  the  illuminating 
pencil  or  cone  of  light.  If  this  is  incorrect,  good  results  cannot  be  ob- 
tained. First  see  that  the  plane  glass  of  the  vertical  illuminator  is  set  an 
angle  of  45°.  Place  a  highly  polished,  unetched  specimen  on  stage  face 
down,  focus  objective,  and  while  observing  in  the  eyepiece  bring  the 
small  spot  of  light  to  the  center  of  the  field.  The  spot  should  appear  as 
the  one  in  Figure  41  marked  correct.  If  it  is  off  center,  the  vertical  il- 
luminator should  be  adjusted  either  by  knob  of  plane  glass  No.  6,  or  by 
rotating  the  illuminator  upon  the  verticle  axis  until  the  spot  is  centrally 

located. 

31 


Going  back  to  Figure  40,  focus  the  objective  roughly  and  remove  eye- 
piece, No.  2.  Place  over  the  eyepiece  tube  a  pinhole  diaphragm,  for  cen- 
tering the  eye  with  tube.  Place  supplementary  lens,  E,  in  position  about 
6  inches  to  right  of  microscope,  and  while  observing  through  pinhole 
cap,  adjust  this  condenser  until  the  bright  spot  is  in  center,  as  just  shown. 
Then  replace  eyepiece  and  close  iris,  No.  16,  of  supplementary  lens.  If 
the  iris  is  not  sharply  defined,  slide  the  supplementary  lens  tm  its  base, 
No.  1 8,  to  or  from  the  microscope  until  the  blades  of  iris  diaphragm  can 
be  distinctly  seen  in  the  field,  when  objective  is  in  focus.  In  making  these 
last  observations,  a  smoked-glass  cap  should  be  used  over  the  eyepiece. 
The  image  of  this  iris  should  be  concentric  with  diaphragm  of  the  eye- 
piece, and  will  be  if  set-up  has  been  correctly  made. 

If  the  arc  is  now  focused  on  the  supplementary  lens,  the  light  condition 
should  be  as  shown  in  Figure  13.  The  arc  is  nearly  focused  on  supple- 
mentary lens  No.  2,  by  lens  No.  i.  Lens  and  iris  No.  i  are  focused  at 
back  of  objective  by  lens  No.  2.  Arc,  lens,  and  iris  No.  2  are  focused  on 
specimen  by  the  objective.  This  gives  critical  illumination.  When  these 
results  are  obtained,  good  images  are  secured.  After  a  little  practice  it  is 
not  difficult  to  set  up  the  apparatus  in  the  manner  described.  Some  users 
introduce  a  ground  glass  in  front  of  the  arc.  This  diffuses  the  light,  and 
while  giving  even  illumination,  it  does  not  give  critical  illumination  as 
previously  described,  but  is  preferred  by  some  for  lower  powers  and  is 
even  recommended  by  some  workers. 

In  adjusting  the  mirror  illuminator,  or  if  a  prism  illuminator  is  used,  the 
same  conditions  prevail.  The  spot  of  light  is  to  be  centered  in  the  field  as 
before,  and  the  supplementary  lens  then  placed  in  position  and  the  loca- 
tion of  the  light  pencil  adjusted  as  shown  in  Figure  42.  In  this  case  it 
will  be  noticed  that  in  the  correct  position  the  circle  of  light  is  located  in 
the  middle  of  the  hemisphere  and  not  centrally  with  the  optical  axis.  The 
mirror  illuminator  is  excellent  for  projecting  with  low  powers.  A  little 
practice  will  enable  one  quickly  to  make  these  adjustments,  and  the  re- 
sults will  amply  repay  for  the  time  spent. 


Fig.  43 — Diagram  showing  illuminating  system  with  short  focus  condenser. 

*The  most  recent  models  (1927-8)  are  arranged,  as  shown  in  Figure  43, 
with  the  source  of  illumination  9^  inches  from  the  axis  of  the  microscope. 
Such  an  arrangement  permits  critical  illumination  and  makes  a  more 

32 


compact  instrument  with  fewer  adjustments.  This  reduction  of  the  dis- 
tance from  the  light  source  to  the  vertical  axis  is  accomplished  by  a  short- 
focus  condenser  and  a  supplementary  lens  to  magnify  and  form  a  virtual 
image  of  the  light  source. 

The  light  source  S  (Fig.  43),  is  imaged  at  S'  under  a  magnification  of 
about  1.5  by  the  short-focus  condenser  A.  B  is  an  auxiliary  condenser 
placed  very  close  to  the  axis  of  the  microscope  and  of  such  focal  fength  as 
to  form  a  real  image  of  the  condenser  A  in  the  back  focal  plane  of  the 
microscope  objective.  The  auxiliary  condenser  B  must  also  form  a  virtual 
image  of  the  light  source  S'  in  the  conjugate  focal  plane  of  the  objective. 
That  is,  the  lens  B  places  the  light-source  image  to  the  left,  at  a  distance 
from  the  objective  equal  to  the  distance  to  the  right  from  the  image 
plane  to  the  objective,  or  at  a  position  such  that  the  objective  will  image 
the  light-source  S  on  the  specimen  O.  Thus,  critical  illumination  results, 
and  we  have  a  short  system  of  illumination  performing  the  same  as  that 
represented  in  Figures  13  and  40.* 

The  elimination  of  vibration  is  one  of  the  difficult  problems  that  most 
metallurgists  have  to  overcome.  No  matter  how  rigid  the  apparatus, 
short  sharp  blows  will  produce  vibrations  which  make  it  impossible  to 
make  good  photomicrographs.  There  are  a  number  of  different  devices 
in  use  ranging  from  simple  to  very  elaborate  appliances.  Each  user  seems 
satisfied  with  his  particular  device.  Probably  the  simplest  and  easiest  to 
install  consists  of  two  planks,  of  a  size  suitable  to  hold  the  entire  appara- 
tus, and  between  them  two  inflated  3^2  inch  inner  tubes  or  two  circles  of 
tennis  balls. 

Another  device  much  in  use  consists  of  a  platform  suspended  from  the 
ceiling,  some  consisting  of  simple  chains  or  ropes,  others  of  springs  ar- 
ranged with  oil  dampers  below  platform  to  check  the  spring  movement. 
Another  device,  designed  by  Mr.  Pafenbach  of  the  Simonds  Stell  Co., 
Lockport,  New  York,  consists  of  a  submerged  platform  level  with  the 
floor,  but  suspended  upon  springs  held  by  angle  pieces  projecting  down- 
ward from  cross  beams,  also  level  with  the  floor.  It  seems  that  each  one 
might  best  provide  such  means  as  are  necessary  in  his  particular  location, 
but  vibration,  if  it  exists,  should  be  eliminated.  Vibration  can  be  detected 
easily  by  observing  the  reflections  from  a  small  dish  of  mercury  set  upon 
the  apparatus. 

A  focusing  glass  is  a  desirable  addition,  and  almost  necessary  if  one  ex- 
pects to  obtain  a  sharp  image  upon  the  photographic  plate,  especially  in 
high  magnifications.  As  to  the  proper  color  of  filter  to  use  in  metallogra- 
phy, there  seems  to  be  some  differences  of  opinion,  no  doubt  due  to  dif- 
ferences in  plates,  illuminants,  objectives,  and  specimens  used  by  dif- 
ferent workers.  Different  colors  have  been  experimented  with,  and  it  has 
been  found  that  on  iron  and  steel  specimens  a  combination  of  green  and 
yellow  dominant  wave  length  of  5500  Angstroms  give  the  best  results. 
The  Angstrom  unit  equals  one  ten-millionth  of  a  millimeter.  Often  the 
appearance  of  the  image  upon  the  ground  glass  is  made  the  basis  of  the 
proper  filter  to  secure  contrast  desired,  but  suitable  plates  must  be  used  to 

33 


Green 


Red 


|GY| 


NORMAL  SPECTRUM  SHOWING  THE  SENSITIVENESS  OF  ORDINARY 
PHOTOGRAPHIC  PLATES. 

(After  Mees,  and  magnified  as  in  fig.  139). 

Fig.  44 — Sensitive  region^of  the  ordinary  photographic  plate. 


X0.6-  «X7- 

NORMAL  SPECTRUM  SHOWING    THE    SENSITIVENESS  OF  ORTHO- 
CHROMATIC  OR' ISOCHROMATIC  PLATES. 

(After  Mees;  magnification  as  in  fig.  139). 

Fig.  45 — Sensitive  region  of  orthochromatic  or  isochromatic  plates. 


NORMAL  SPECTRUM  SHOWING  THE  SENSITIVENESS  OF  PANCHRO- 
MATIC PLATES. 

(After  Mees;  magnification  as  in  fig.  139). 

Fig.  46 — Sensitive  region  of  panchromatic  plate J ror  a  wide  range  of  the  spectrum. 

record  such  an  image.  Much  difficulty  is  found  in  obtaining  solid  glass 
filters  that  will  give  the  color  desired,  so  it  becomes  necessary  to  use  the 
glass-mounted,  gelatine-stained  variety.  In  the  matter  of  plates  one 
must  select  a  plate  suitable  to  the  work  and  filter  used. 

Figure  44  shows  the  sensitive  region  of  the  ordinary  plate,  and  if  filters 
are  used  which  transmit  only  green  and  yellow  with  very  little  blue,  the 
exposure  will  be  very  long  and  the  results  not  satisfactory.  Figure  45 
shows  the  sensitive  region  of  the  orthochromatic  or  isochromatic  plates, 
and  if  filters  are  used,  such  as  the  B  &  G  Wratten,  transmitting  wave 
lengths  around  5500,  this  type  of  plate  gives  good  results.  The  slower 
orthochromatic  plates  have  a  finer  grain  and  are  therefore  best  for 
photomicrographic  work.  Such  plates  as  the  Stanley  Commercial  or  the 
Cramer  Slow  Iso  will  give  good  results.  If  heat-tinted  specimens  are 
used  and  there  is  much  variation  in  colors,  all  of  which  one  may  wish  to 
record,  then  the  panchromatic  plate  should  be  used.  Figure  46  shows 
that  these  plates  are  sensitive  to  a  wide  range  of  the  spectrum. 

The  many  factors  involved,  such  as  objectives,  aperture,  magnification, 
illuminant,  filter,  etc.,  make  the  keeping  of  records  almost  imperative.  A 
method  of  standardizing  a  photomicrographic  outfit  has  been  described 
by  Prof.  Alexander  Petrunkevitch  of  Yale  University  in  the  Anatomical 
Record,  Vol.  19,  No.  5,  October  1920.  While  his  article  refers  particularly 
to  transparent  photography,  the  same  rules  can  be  applied,  namely,  to 

34 


determine  the  best  location  for  each  and  every  part  of  the  apparatus,  dia- 
phragm apertures,  for  different  objectives,  ray  filters,  plates,  etc.,  and 
after  establishing  such  record,  one  may  make  a  picture  of  a  given  kind  of 
object  in  the  minimum  amount  of  time  and  be  practically  sure  of  the  re- 
sult without  recourse  to  cut  and  try  methods  every  time  a  picture  is  to  be 
made. 

The  surface  of  the  polished  specimen  must  be  flat.  Any  rocking  on  the 
grinding  or  polishing  wheel  will  produce  a  cylindrical  surface  and  the 
field  in  the  microscope  will  have  a  band  in  focus  while  both  sides  may  be 
out  of  focus.  Also,  the  etching  must  be  done  properly  to  produce  the  con- 
trast necessary  to  show  the  structure  desired.  Light  etching  has  been 
found  to  give  the  best  results  with  high  powers.  It  is  well  again  to  em- 
phasize the  necessity  of  keeping  all  parts  of  the  apparatus  clean,  not  only 
all  lenses,  but  all  bearing  surfaces.  The  apparatus  should  be  protected 
with  a  rubber  cloth  covering  when  not  in  use.  The  instrument  should  be 
placed,  if  possible,  where  it  is  not  exposed  to  the  fumes  of  the  chemical 
laboratory. 


35 


Table  of  Magnifications  For  ILS  Inverted  Microscope 


OBJECTIVES 

Eyepiece 
Power 

Distance  from  eyepiece  to  screen  in  CM 

E.F.  in  MM      Magnification 

25 

50 

75 

100 

Achromat 
32 

ic  System 
5.2 

5.0 
6.4 
7.5 
10.0 
12.5 
15.0 

25 
33 
39 
52 
65 
78 

52 
66 
78 
104 
130 
156 

78 
99 
117 
156 
195 
234 

104 
132 
156 
208 
260 
312 

16 

12.5 

5.0 
6.4 
7.5 
10.0 
12.5 
15.0 

62 
80 
94 
125 

156 

188 

124 
160 
188 
250 
312 
376 

186 
240 
282 
375 
468 
564 

248 
320 
376 
500 
624 
752 

8 

26 

5.0 
6.4 
7.5 
10.0 
12.5 
15.0 

130 
166 
195 
260 
325 
390 

260 
332 
390 
520 
650 
780 

390 
498 
585 
780 
975 
1170 

520 
664 
780 
1040 
1300 
1560 

4 

52.3' 

5.0 
6.4 
7.5 
10.0 
12.5 
15.0 

262 
335 
392 
523 
654 
785 

524 
670 
784 
1046 
1308 
1570 

786 
1005 
1176 
1569 
1962 
2355 

1048 
1340 
1568 
2092 
2616 
3140 

1.9 

116.5 

5.0 
6.4 
7.5 
10.0 
12.5 
15.0 

583 
746 

874 
1165 
1456 

1748 

1166 
1492 
1748 
2330 
2912 
3496 

1749 
2238 
2622 
3495 
4368 
5244 

2332 
2984 
3496 
4660 
5824 
6992 

Fluorite 
4 

System 
51.6 

5.0 
6.4 
7.5 
10.0 
12.5 
15.0 

258 
330 
387 
516 
645 
774 

516 
660 
770 
1032 
1290 
1548 

774 
990 
1161 
1548 
1935 
2322 

1032 
1320 
1548 
2064 
2580 
3096 

1.8 

120 

5.0 
6.4 
7.5 
10.0 
12.5 
15.0 

600 
768 
900 
1200 
1500 
1800 

1200 
1536 
1800 
2400 
3000 
3600 

1800 
2304 
2700 
3600 
4500 
5400 

2400 
3072 
3600 
4800 
6000 
7200 

Apochrom 
8 

atic  System 

28 

5.0 
6.4 
7.5 
10.0 
12.5 
15.0 

140 
179 
210 
280 
350 
420 

280 
358 
420 
560 
700 
840 

420 
537 
630 
840 
1050 
1260 

560 
716 
840 
1120 
1400 
1680 

4 

58.2 

5.0 
6.4 
7.5 
10.0 
12.5 
15.0 

291 
372 
436 
582 

728 
873 

582 
744 
872 
1164 
1456 
1746 

873 
1116 
1308 
1746 
2184 
2619 

1164 
1488 
1744 
2328 
2912 
3492 

3 

74 

5.0 
6.4 
7.5 
10.0 
12.5 
15.0 

370 
477 
555 
740 
925 
1110 

740 
948 
1110 
1480 
1850 
2222 

1110 
1422 
1665 
2220 
2775 
3330 

1480 
1896 
2220 
2960 
3700 
4440 

2 

111 

5.0 
6.4 
7.5 
10.0 
12.5 
15  0 

555 
710 

833 
1110 
1388 
±  1  fifiS 

1110 
1420 
1666 
2220 
2776 
3330 

1665 
2130 
2499 
3330 
4164 
4PPS 

2220 
2840 
3332 
4440 
5552 
6660 

NOTE:    Above  objectives' are  short  mounted  and  corrected  for  215  mm  tube  length. 
The  first  column  of  magnifications  represent  visual  magnifications  as  well  a 
25  cm. 


Form  No.  E-214— VIII— 28 


36 


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Patterson,  W.L. 

The  optics  of  metallography, 


